1. Field of the Invention
This invention relates to methods and apparatus for etching a workpiece using Xenon Difluoride (XeF2). Xenon Difluoride is a dry isotropic gas phase etchant, which provides a gentle etch for silicon at low temperature. Xenon Difluoride is usually supplied in the form of colourless crystals which sublime without decomposition. The sublimation pressure for XeF2 is approximately 4 Torr.
2. Description of the Background Art
Present attempts to use XeF2 for etching have been essentially experimental and have taken place using a pulsed supply of XeF2 which requires the etch to be stopped and started with the etch chamber being pumped down between each etch step. Such an arrangement is impracticable for production processes. Direct flow has been attempted unsuccessfully.
From one aspect the invention consists in a method of etching a workpiece using XeF2, including allowing XeF2 in its solid phase to sublimate into its gaseous state into a reservoir of sufficient volume to provide gas at a pre-determined flow rate for a pre-determined etch period, supplying the gas at the desired flow rate to an etching chamber containing the workpiece and etching the workpiece.
The XeF2 gas may be mixed with an inert carrier gas prior to its introduction into the etch chamber. It is particularly preferred that the XeF2 source continues to sublimate during the outward flow of XeF2 from the reservoir. Additionally or alternatively the reservoir may be re-charged between the etching of separate workpieces.
From a further aspect the invention consists in apparatus for etching a workpiece including, an etched chamber, a XeF2 source, a reservoir, a valve for connecting the source to the reservoir to allow sublimation of the source into XeF2 gas, a flow controller for feeding the etch chamber and a valve for connecting the reservoir to the flow controller.
Preferably the apparatus includes pressure controlles means for maintaining the reservoir at approximately the sublimation pressure of XeF2 when there is no outward flow from the reservoir. Means may be provided for mixing the XeF2 gas with an inert carrier gas prior to its introduction into the process chamber. It is particularly preferred that is the flow controller is a pressure-based flow controller.
A chamber will normally be provided for the solid XeF2 and conveniently the reservoir may have a volume which is approximately three times the volume of the XeF chamber.
Although the invention has been described above, it is to be understood that it includes any inventive combination of the features set out above or in the following description.